Mechanism Of Internal Tide Generation And Seasonal Variation Of Internal Tides Around The Luzon Strait

The generation and evolution of internal tides over the topography is closelyrelated to topography, stratification and barotropic tide, which have been studied byusing analytical and numerical calculations. Furthermore, seasonal variations ofinternal tides around the Luzon Stait and their relation with mixing in the basin ofSCS have been discussed.Firstly, internal tide generated over weak topographies was studied. Resultsobtained from the analytical solution show that tide-topography resonance takes placewhen the topography wavenumber is equal to that of one baroclinic mode (theresonant mode). For resonant modes, the amplitudes increase with topography length.For non-resonant modes, the amplitudes are weak and vary periodically with theextending of the topography. The numerical results show that phase of theperturbation pressure gradually agrees with the phase of topography with the increaseof topography length for resonant modes, and the conversion rate is always positiveover the topography, thus the resonant mode persists in absorbing energy from thebarotropic tide, and the conversion rate is increased. Otherwise, phase of theperturbation pressure gradually becomes more and more irregular with the increase oftopography length for non-resonant modes, the conversion rate in the middle of thetopography is approaching zero, negative conversion rates appear on some sites, sointernal tides can not absorb energy from tide topography interaction.As the baroclinic modes propagate, they continue to interact with the bottom topography, scattering might take place to convert one baroclinic mode to otherbaroclinic modes. Furthermore, it is shown that the interaction between internal tideand bottom topography can lead to not only scattering but also resonance, transferringenergy into other baroclinic modes. When none of the conditions for resonance andscattering are satisfied, the modes of incident internal tides will not be changed by theinteraction with the topography.Over the high single gaussian topography, baroclinic velocity field change fromlow mode structure to ray-like structure as the topography becomes higher andnarrower. Over the low and wide topography, the structure of baroclinic velocity fielddoes not change obviously with the strengthen of barotropic tides; while over high andnarrow topography, with the strengthen of barotropic tides, nonlinear effects areenhanced, and the structure of baroclinic velocity field becomes disordered. Whenboth of the western and eastern guassian topographies are present and the barotropictides are strong, the generation of westward propogating internal solitary waves willbe enhanced due to interactions with the western topography.Seasonal variations of baroclinic tides for K1and M2constituents wereseparately studied using two-dimensional numerical simulations along the21°Nsection of the northern South China Sea (SCS). Results show that the continentalslope of the northern SCS and the west ridge of the Luzon Strait are supercritical toK1internal tides, which may be trapped in the deep basin of the SCS and formstanding or partial standing waves. Meanwhile, these areas are sub-critical to M2internal tides, which can transmit onto the shelf and are seldom reflected back into thebasin. The trapped K1internal tides are dominated by mode-2and mode-3in summer and by mode-1and mode-3in winter. Moreover, high mode K1internal tides accountfor nearly20%to40%of the total energy density in winter and15%to20%insummer. The pattern of K1internal tides in the basin is mainly determined by thepercentage of reflected energy from the continental slope. The phase differencebetween the incoming mode-1and mode-2K1internal tides near the continental slopeare nearly out of phase in winter, which means the percentage of reflection of the K1internal tide is larger than that in summer. Both the convergence and high mode K1internal tides can enhance the vertical shear. The above results indicate that, in thedeep basin of the SCS, water mixing potentially induced by internal tides in winter isstronger than in summer.The3-D model simulation for the internal tides in the northern South China Seashows that Luzon strait is the most important source site for internal tides with90%generation of total K1and M2internal tides. The major source site for M2internaltide is the west ridge in the LS, for K1internal tides, it is in the middle of the eastridge. The seasonal variation of the generation of K1and M2internal tides is relatedwith the joint action of the stratification and topography. If the stratification isfavarable for the resonance, the barotropic to baroclinic tide conversion rate will beincreased. Otherwise, if the stratification weakens the resonance, the barotropic tobaroclinic tide conversion rate may be decreased, even if the stratification isenhanced.Luzon Strait is also the main dissipation area for the internal tides. It is estimatedthat about33%of K1intenral tides and50%of M2intenral tide are dissipated locallythere. One branch of the M2barotropic tide propagating into the South China Sea spreads toward the eastnorth direction and interacts with the continental shelf at thewestsouth of Taiwan. Hence, the generation and dissipation of M2internal tides isvery obvious over the continental shelf of the westsouth of Taiwan. As a whole, in thestudy area, the generation and dissipation of K1and M2internal tides is stronger inwinter than that in summer.The topography with double ridges in the Luzon Strait is favorable for theformation of standing and partial standing waves during the propogation of K1andM2internal tides there. For the entire study area, the standing and partial standingwaves in winter spreads more widely than in summer. In the basin of SCS, thestanding and partial standing waves for the first mode of K1internal tide mainlydistributes around the basin, while the second and the third modes of K1internal tidesis mainly in the middle of the basin. The standing and partial standing waves for thesecond mode of M2internal tide distributes more widely than that for the first modeof M2internal tide.